In order that the braking forces are more ideally distributed to the front and rear wheels of a passenger vehicle, the disc brake is increasingly widely used for the front and rear wheels. But the existing disc brake has three shortages: 1) the parking requires a large displacement of the cable; 2) the effective life of parking is short; and 3) the part cost is high.
The conventional disc brake is illustrated in FIG. 1. During parking, the cable pulls a parking lever 21, and a thrust shaft 22 is rotated along with the parking lever. When the thrust shaft 22 is rotated, the thrust shaft 22 is fitted with a ball 24 due to a ramp structure of the bottom of the thrust shaft 22. The ball 24 rolls upward to the ramp of the thrust shaft 22. Similarly, a thrust nut 25 fitted with the ball 24 at the other end also has a bottom in the form of a ramp structure. The ball 24 rolls upward to the ramp of the thrust shaft 22, and also rolls upward to the ramp of the thrust nut 25 at the same time, thus the thrust nut 25 bears the axial pressure and the rotation torque simultaneously. The thrust nut 25 is mounted in an anti-rotation frame 23 and is prevented to rotate by the anti-rotation frame 23. The anti-rotation frame 23 is mounted in a brake housing 26 and is prevented to rotate by the brake housing 26. The thrust nut 25 transfers an axial pressure to an adjusting bolt 27, the adjusting bolt 27 transfers the pressure to a piston 28, the piston 28 transfers the pressure to a friction pad 29, and the friction pad 29 applies a pressure to the brake disc to prevent a rotation of the brake disc, thereby achieving a parking effect.
In the conventional disc brake, theoretically the thrust nut 25 is prevented from rotating completely by the anti-rotation frame 23. But in practice, due to the part machining tolerance, there is a gap A between the thrust nut 25 and the anti-rotation frame 23 as illustrated in FIGS. 2-3, and there is a gap B between the anti-rotation frame 23 and the brake housing 26 as illustrated in FIGS. 4-5. When the cable starts to pull the parking lever 21, the thrust nut 25 is rotated for some extent due to the existence of the gaps A and B. As a result, the displacement of the cable required by the parking is increased, and the parking efficiency is decreased.